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Organization and Business - (Narrative) (Details)
ft² in Millions
12 Months Ended
Dec. 31, 2019
ft²
room
property
building
unit
Real estate fund investments  
Real Estate Properties  
Equity method ownership percentage 25.00%
Alexanders Inc  
Real Estate Properties  
Equity method ownership percentage 32.40%
New York | Manhattan | Office Building  
Real Estate Properties  
Square footage of real estate property (in sqft) 19.1
Number of real estate properties | property 35
New York | Manhattan | Retail  
Real Estate Properties  
Square footage of real estate property (in sqft) 2.3
Number of real estate properties | property 70
New York | Manhattan | Residential Properties  
Real Estate Properties  
Number of real estate properties | property 10
Number of units in real estate property (in number of residential units) | unit 1,991
New York | Manhattan | Hotel Pennsylvania  
Real Estate Properties  
Number of units in real estate property (in number of residential units) | room 1,700
New York | Alexanders Inc | New York City Metropolitan Area  
Real Estate Properties  
Number of real estate properties | property 7
Equity method ownership percentage 32.40%
New York | Alexanders Inc | New York City Metropolitan Area | Lexington Avenue 731  
Real Estate Properties  
Square footage of real estate property (in sqft) 1.3
Other | Real estate fund investments  
Real Estate Properties  
Equity method ownership percentage 25.00%
Other | Chicago | the Mart, Chicago  
Real Estate Properties  
Square footage of real estate property (in sqft) 3.7
Other | San Francisco | 555 California Street | Office Building  
Real Estate Properties  
Square footage of real estate property (in sqft) 1.8
Equity method ownership percentage 70.00%
Number of buildings | building 3
Operating Partnership  
Real Estate Properties  
Common limited partnership interest in the Operating Partnership 93.10%